Composite metallic ultrafine particles and process for producing the same

ABSTRACT

The present invention relates to composite metallic ultrafine particles which have excellent dispersion stability and can be produced on an industrial scale, and a process for producing the same, and a method and an apparatus for forming an interconnection with use of the same. A surface of a core metal produced from a metallic salt, a metallic oxide, or a metallic hydroxide and having a particle diameter of 1 to 100 nm is covered with an organic compound including a functional group having chemisorption capability onto the surface of the core metal.

This is a division of parent application Ser. No. 09/811,581, filed Mar.20, 2001, now U.S. Pat. No. 6,743,395.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to composite metallic ultrafine particlesand a process for producing the same, and more particularly to compositemetallic ultrafine particles which have excellent dispersion stabilityand can be produced on an industrial scale, and a process for producingthe same, and a method and an apparatus for forming an interconnectionwith use of the same.

2. Description of the Related Art

Generally, as a process for producing metallic ultrafine particleshaving a diameter of not more than 100 nm, there has been known aprocess in which metal is evaporated under vacuum in the presence of asmall amount of gas, and then metallic ultrafine particles are condensedfrom the vapor phase to obtain ultrafine metallic particulates. However,this physical process suffers from practical problems that {circlearound (1)} the variation in the particle diameter distribution is solarge that even if a heating process is performed for film formation,grain boundaries are left, and a uniform metallic film cannot beobtained, {circle around (2)} the amount of metallic ultrafine particlesproduced in a single operation is so small that this process is notsuitable for mass production, and {circle around (3)} devices for anelectron beam, plasma, laser, induction heating, or the like arenecessary for the evaporation of metal, and hence the production costrises, for example.

Further, when the metallic ultrafine particles are solely taken out intothe air, they are agglomerated. Therefore, it is necessary to dispersethe particles in a solvent with use of a surface-active agent or thelike. However, since the dispersion stability is poor, the storagestability is unsatisfactory.

There has been reported a method for producing metallic ultrafineparticles in which metal ions produced from a metallic salt in anaqueous solvent are stabilized by a polymeric protective agent. However,such metallic ultrafine particles are limited to being handled in anaqueous system and thus have poor flexibility. Further, in this case, ahigh-molecular weight dispersant should be used for stabilization ofmetallic ultrafine particles. Therefore, metal content is lowered, andthe ultrafine particles have little expectation of using as a metalsource.

Metallic ultrafine particles have high activity and are unstable.Therefore, when such metallic ultrafine particles are gathered in a bareparticle state, the particles are easily adhered to each other to causean agglomeration or to be chained. In order to stabilize metallicultrafine particles in such a state that the particles are separatedfrom each other, it is necessary to form a certain protective coating onthe surface of the metallic ultrafine particles. Further, in order thatmetallic ultrafine particles are stable as particles even with a highmetal content, a metallic core should be stably bonded to the protectivecoating formed therearound.

When the production of metallic ultrafine particles on an industrialscale is taken into consideration, metallic ultrafine particles shouldpreferably be safety and simply produced in a production process, andsuch metallic ultrafine particles are required to be utilized in variousfields and to have good flexibility.

The present inventors have found the following thing. A certain metallicsalt, a metallic oxide, or a metallic hydroxide is mixed with an organiccompound including a functional group having chemisorption capabilityonto a metal produced from the metallic salt, the metallic oxide, or themetallic hydroxide. In a process in which the mixture is heated underthe reflux condition of the organic compound for reaction, a core of anultrafine particle of pure metal is produced by pyrolysis of themetallic salt, the metallic oxide, or the metallic hydroxide. Theorganic compound is chemisorbed onto the core by the functional group ofthe organic compound to form composite metallic ultrafine particleshaving a stable protective coating with high efficiency.

Further, it has been found that since a metallic salt, a metallic oxide,or a metallic hydroxide as a metal source and an organic compound for aprotecting coating are different from each other, there is a possibilitythat the metal content, the reactivity, the particle diameter, or thelike of the composite metallic ultrafine particles to be produced can becontrolled by varying the combination of the metal source with theorganic compound. Further, since the amount ratio of the metal source tothe organic compound can also be manipulated as desired, processes fromsynthesis to purification of the composite metallic ultrafine particlescan easily be optimized.

The present inventors have found that, in a process in which a certainmetallic salt is mixed with an organic compound including an alcoholichydroxyl group and pyrolyzed, an alcohol is bonded to the periphery of acore metal with a metal alkoxide bond to form composite metallicultrafine particles having a stable protective coating. Further, thepresent inventors have invented a novel method for forming aninterconnection on a semiconductor substrate with use of this compositemetallic ultrafine particle, and an apparatus using this method.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above circumstances.It is therefore an object of the present invention to provide compositemetallic ultrafine particles which have more uniform particle diametersand have excellent dispersion stability and enhanced stability in theproperties of particles, and a process for producing composite metallicultrafine particles which can produce such composite metallic ultrafineparticles simply and stably on an industrial scale.

According to a first embodiment of a composite metallic ultrafineparticle of the present invention, there is provided a compositemetallic ultrafine particle characterized in that a surface of a coremetal produced from a metallic salt, a metallic oxide, or a metallichydroxide and having a particle diameter of 1 to 100 nm is covered withan organic compound including a functional group having chemisorptioncapability onto the surface of the core metal.

It has been known that that the melting point of metallic particleslowers as the particle diameter decreases. This effect appears when theparticle diameter is not more than 100 nm, and becomes significant whenthe particle diameter is 1 to 18 nm. When the particle diameter is 1 to10 nm, some metals begin to melt even at around ordinary temperature.Therefore, the average particle diameter of the core metal (metallicultrafine particle) is preferably 1 to 50 nm, and more preferably 1 to18 nm, and, particularly, more preferably 1 to 10 nm. Further, thesurface of the core metal is stably covered with the organic compoundstrongly chemisorbed onto the surface of the core metal with a chemicalbonding strength. This organic compound serves as a protective coatingfor protecting the core metal, for thereby improving dispersionstability in a solvent and stability in the properties of particles. Asthe particle diameter of the core metal decreases, the proportion of theprotective coating relatively increases to lower the metal content.Therefore, for some applications, it is not advisable to excessivelyreduce the particle diameter of the metal portion.

Here, chemisorption capability refers to formation of chemical bond onlyto the surface of the object without involving any further reaction.Thus, chemisorption capability is different from chemical reaction whichimplies reaction with the surface of the object so as to cut the bondbetween the surface atoms and the inside of the object material and tofinally remove the surface atoms from the surface of the object.

Specifically, for example, in the case of copper, when a carboxylic acidis used as a protective group, as shown in the following scheme, thecarboxylic acid is chemisorbed on the surface of copper at a lowtemperature, and, at a certain high temperature, a reaction occurs todisadvantageously remove surface atoms of copper and to form a coppersalt of the carboxylic acid. This is considered attributable to the factthat, at such a high temperature, the carboxylic acid-copper bondingstrength is stronger than the copper-copper bonding strength. Thus, inthis case, the decomposition effect is superior to the protectiveeffect, and, accordingly, when ultrafine particles of copper have beenformed, even when copper ultrafine particles are formed, they areimmediately decomposed to a copper salt of the carboxylic acid.

On the other hand, for example, when a higher alcohol having five ormore carbon atoms is used as a protective group, as shown in thefollowing scheme, the alcohol is chemisorbed onto the surface of copperto form a monomolecular layer, which, even under considerably hightemperature conditions, causes no more reaction and can serve as aprotective layer, because the reactivity of the higher alcohol is not ashigh as that of the carboxylic acid.

In the case of silver, both of the carboxylic acid and the higheralcohol have relatively low chemical reactivity compared with stabilityof silver. Hence, when the carboxylic acid and the higher alcohol areused as a protective group, these compounds are chemisorbed onto thesurface of silver with no more reaction with silver. According to thepresent application, as described above, a combination of a metallicsalt, a metallic oxide, or a metallic hydroxide as a metal source withan organic compound for the protective coating can be selected asdesired. Therefore, as exemplified above, the protective coating can beoptimized according to the properties of the selected metal.

As the core metal, there may be used at least one member selected fromthe group consisting of Ag, Au, Bi, Co, Cu, Cr, Fe, Ge, In, Ir, Ni, Os,Pd, Pt, Rh, Ru, Si, Sn, Ti, and V, for example. It is desirable that theamount of organic compound for covering the core metal is 0.01 to 1molecule, per metal atom on the surface of the core metal.

As the organic compound, there may be used an alcoholic hydroxyl group,carboxyl, thiol, amino, or amide group which has four or more carbonatoms.

As the metallic salt, there may be used carbonate, nitrate, chloride,acetate, formate, citrate, oxalate, urate, phthalate, or a fatty acidsalt having four or less carbon atoms.

According to a first embodiment of a process for producing compositemetallic ultrafine particles of the present invention, there is provideda process for producing composite metallic ultrafine particles,characterized by comprising: mixing a metallic salt, a metallic oxide,or a metallic hydroxide with an organic compound including a functionalgroup having chemisorption capability onto a surface of a core metalproduced from the metallic salt, the metallic oxide, or the metallichydroxide; and heating the mixture for reaction.

According to a second embodiment of a process for producing compositemetallic ultrafine particles of the present invention, there is provideda process for producing composite metallic ultrafine particles,characterized by comprising: mixing a metallic salt, a metallic oxide,or a metallic hydroxide with an organic compound including a functionalgroup having chemisorption capability onto a surface of a core metalproduced from the metallic salt, the metallic oxide, or the metallichydroxide; and heating the mixture under a reflux condition of theorganic compound for reaction.

The above composite metallic ultrafine particles can be produced in achemical process, and, therefore, can be mass-produced with use of asimple apparatus, without use of a large vacuum apparatus, in a usualatmospheric atmosphere. This contributes to lowered cost.

According to a second embodiment of a composite metallic ultrafineparticle of the present invention, there is provided a compositemetallic ultrafine particle having a structure in which a periphery of acore metal having a diameter of 1 to 100 nm is surrounded by an organiccompound including an alcoholic hydroxyl group.

It has been known that that the melting point of metallic particleslowers as the particle diameter decreases. This effect appears when theparticle diameter is not more than 100 nm, and becomes significant whenthe particle diameter is not more than 20 nm. The melting point islargely lowers when the particle diameter is not more than 10 nm.Therefore, from the viewpoint of use, the average particle diameter ofthe core metal (metallic ultrafine particle) is preferably 1 to 20 nm,and more preferably 5 to 15 nm. Further, the organic compound includingan alcoholic hydroxyl group serves as a protective coating forprotecting the core metal. It is advantageous, for example, in that, inuse of the metallic ultrafine particles, the composite metallicultrafine particles can easily be decomposed at a low temperaturewithout significant obstruction by the protective coating. Furthermore,this can improve dispersion stability in a solvent and stability in theproperties of particles.

The organic compound including an alcoholic hydroxyl group may be astraight-chain or branched-chain alcohol having four or more carbonatoms, or an aromatic compound including a hydroxyl group.

According to a third embodiment of a process for producing compositemetallic ultrafine particles of the present invention, there is provideda process for producing composite metallic ultrafine particles,characterized by comprising heating an organic compound including analcoholic hydroxyl group and a metallic salt as a metal source at atemperature that is not more than a decomposition initiation temperatureof the organic compound including an alcoholic hydroxyl group and is notless than a decomposition temperature of the metallic salt.

According to a fourth embodiment of a process for producing compositemetallic ultrafine particles of the present invention, there is provideda process for producing composite metallic ultrafine particles,characterized in that: a reducing agent, such as acetaldehyde,propionaldehyde, or ascorbic acid, in addition to an organic compoundincluding an alcoholic hydroxyl group is added to a metallic salt as ametal source, and the mixture is heated to reduce the metallic salt.

The metal source may be at least one member selected from the groupconsisting of Cu, Ag, Au, In, Si, Ti, Ge, Sn, Fe, Co, Ni, Ru, Rh, Pd,Os, Ir, Pt, V, Cr, and Bi.

The above composite metallic ultrafine particles can be produced in achemical process in a liquid phase, and, therefore, can be mass-producedwith use of a simple apparatus, without use of a large vacuum apparatus.This contributes to lowered cost. Further, since the composite metallicultrafine particles can be produced at a low temperature, energyconsumption can be reduced to contribute lowered cost. Furthermore,since the raw materials used are harmless to the environment, thecomposite metallic ultrafine particles can safely be produced on anindustrial scale. Moreover, metallic ultrafine particles having auniform diameter can be obtained in a nonaqueous system, and thus can beexpected to be utilized in various fields.

According to a fifth embodiment of a process for producing compositemetallic ultrafine particles of the present invention, there is provideda process for producing composite metallic ultrafine particles,characterized by comprising: dissolving or dispersing a metal source ina hydrophilic nonaqueous solvent to prepare a solution for compositemetallic ultrafine particles; adding, to a hydrophobic nonaqueoussolvent, an organic compound including a functional group havingchemisorption capability onto a surface of a core metal produced fromthe metal source, and the solution for composite metallic ultrafineparticles to prepare a precursor of ultrafine particles; and adding areducing agent to reduce the precursor of ultrafine particles.

The above composite metallic ultrafine particles can be produced in achemical process in a liquid phase, and, therefore, can be mass-producedwith use of a simple apparatus, without use of a large vacuum apparatus,in a usual atmospheric atmosphere. This contributes to lowered cost.Further, since the raw materials used are harmless to the environment,loads on the environment can be small. Moreover, metallic ultrafineparticles having a uniform diameter can be obtained in a nonaqueoussystem, and thus can be expected to be utilized in various fields.

An antioxidant may be added to the solution for composite metallicultrafine particles to produce composite metallic ultrafine particleshaving enhanced stability. Even if a metal susceptible to oxidation isused, the addition of the antioxidant enables the composite metallicultrafine particles to be synthesized. Further, the stability of thecomposite metallic ultrafine particles can also be increased, and it ispossible to store the composite metallic ultrafine particles for a longperiod of time. As the antioxidant, ascorbic acid or vitamin E may beused, for example.

The metal source may be at least one member selected from the groupconsisting of inorganic metallic salts and organometallic compounds(including complexes). It is desirable to use a metal source that isreduced at a temperature which is not more than the boiling point of thehydrophobic nonaqueous solvent in the presence of the reducing agent.

The metal constituting the metallic core may be at least one memberselected from the group consisting of Cu, Ag, Au, In, Si, Ti, Ge, Sn,Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, V, Cr, and Bi.

Hydrophilic nonaqueous solvents may be, for example, alcohols havingfive or less carbon atoms, such as methanol and ethanol, and ketonessuch as acetone. Particularly, alcohols having five or less carbonatoms, such as methanol and ethanol are preferable. The amount (weightratio) of hydrophilic nonaqueous solvent is preferably 1 to 40% withrespect to the hydrophobic nonaqueous solvent.

The organic compound including a functional group having chemisorptioncapability onto a surface of the core metal may be at least one memberselected from the group consisting of higher alcohols having six or morecarbon atoms and surface-active agents, for example.

Citric acid or ascorbic acid may be used as the reducing agent, and thesystem may gradually be heated to a temperature at which the reductionaction is developed.

The hydrophobic nonaqueous solvent may be at least one member selectedfrom the group consisting of petroleum hydrocarbons, such as toluene andxylene, and terpenes, such as terpineol and turpentine. Particularly,petroleum hydrocarbons, such as toluene and xylene, are preferable.

As described above, composite metallic ultrafine particles according tothe five embodiments, the melting point of the composite metallicultrafine particles is extremely lowered, and a metallic film can beformed by a method similar to a type of baking finish. A novel methodfor forming an interconnection on a semiconductor substrate with use ofthis process has advantages with respect to an apparatus, the cost, andloads on the environment, unlike conventional vacuum evaporation,sputtering, and immersion plating.

A process according to the present invention basically comprises coatingcomposite metallic ultrafine particles dispersed in a solvent on asemiconductor substrate on which a fine cavity for an interconnection isformed, drying and baking to form a metallic film, then polishing thesurface of the substrate to form the interconnection, and cleaning anddrying.

According to an embodiment of an apparatus for forming aninterconnection of the present invention, there is provided an apparatusfor forming an interconnection, characterized by comprising: aloading/unloading section having an inlet/outlet port; a dispersionliquid supply device for supplying a dispersion liquid of compositemetallic ultrafine particles to a surface of a substrate, the dispersionliquid of composite metallic ultrafine particles being prepared bydispersing, in a predetermined solvent, the composite metallic ultrafineparticles in which a surface of a core metal is covered with an organiccompound including a functional group having chemisorption capabilityonto the surface of the core metal; a heating device for heating thesubstrate to melt the metal particles and bond them to each other; apolishing device for polishing the surface of the substrate to remove anexcessively deposited metal, and a cleaning/drying device for cleaningand drying the polished substrate.

Preferably, the apparatus further comprises a supplementary dryingdevice for drying a solvent contained in the dispersion liquid ofcomposite metallic ultrafine particles which has been supplied to thesurface of the substrate. The supplementary drying device can completelydry the solvent which has not been dried by simply spin-drying(air-drying) conducted in spin-coating, for example, so that formationof voids can be prevented during a heating process.

Preferably, the apparatus further comprises a bevel/backside cleaningdevice for cleaning a peripheral portion and/or a backside surface ofthe polished substrate.

Preferably, the apparatus further comprises a sensor for measuring afilm thickness in at least one of times after evaporation of a solventcontained in the dispersion liquid of composite metallic ultrafineparticles which has been supplied to the surface of the substrate, aftera heating process in the heating device, and during or after a polishingprocess in the polishing device.

The sensor for measuring a film thickness may be provided in a substrateholding portion in a substrate transfer device for transferring asubstrate. This can eliminate the needs for stop or interruption ofprocessing the substrates and can increase throughput.

It is desirable that pressures in an indoor facility are respectivelycontrolled in a cleaning division having the loading/unloading sectionand a cleaning/drying section housing the cleaning/drying device, and atreatment division having a dispersion liquid supply section having thedispersion liquid supply device therein, a heating section housing theheating device, and a polishing section housing the polishing device;and a pressure in the cleaning division is controlled so as to be higherthan a pressure in the treatment division.

The treatment division in which chemical mist or gas due to chemicalsused for each of the processes is dispersed, and the cleaning divisionfor which a clean atmosphere is required are separated from each otherso that the pressure in the cleaning division is controlled to be higherthan that in the treatment division for preventing the air from flowinginto the cleaning division from the treatment division. Hence, thechemical mist or the gas can be prevented from being attached to thesubstrate after formation of an interconnection.

According to an embodiment of a method for forming an interconnection ofthe present invention, there is provided a method for forming aninterconnection, characterized by comprising: providing a substratehaving a fine cavity formed on a surface of the substrate; supplying adispersion liquid of composite metallic ultrafine particles to thesurface of the substrate, the dispersion liquid of composite metallicultrafine particles being prepared by dispersing, in a predeterminedsolvent, the composite metallic ultrafine particles in which a surfaceof a core metal is covered with an organic compound including afunctional group having chemisorption capability onto the surface of thecore metal; heating the substrate to melt the metal particles and bondthem to each other; polishing the surface of the substrate to remove anexcessively deposited metal; and cleaning and drying the polishedsubstrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an example of production of compositemetallic ultrafine particles according to the present invention;

FIGS. 2A and 2B are diagrams schematically showing a composite metallicultrafine particle according to the present invention;

FIGS. 3A through 3F are diagrams showing, in order of processes, amethod of forming an interconnection with use of composite metallicultrafine particles;

FIG. 4 is a perspective view of an apparatus for forming aninterconnection;

FIG. 5 is a diagram showing an example in which the apparatus forforming an interconnection is disposed in a clean room;

FIG. 6 is a plan arrangement view of the apparatus for forming aninterconnection;

FIG. 7 is a perspective view showing a dispersion liquid supply devicein the apparatus for forming an interconnection, which is partly cutaway;

FIG. 8 is a vertical cross-sectional view of the dispersion liquidsupply device in the apparatus for forming an interconnection;

FIG. 9 is a cross-sectional view of a supplementary drying device in theapparatus for forming an interconnection;

FIG. 10 is a schematic view of a heating device in the apparatus forforming an interconnection;

FIG. 11 is a vertical cross-sectional view of the heating device in theapparatus for forming an interconnection;

FIG. 12 is a rear view of a housing of the heating device in theapparatus for forming an interconnection;

FIG. 13 is a plan view of a heating plate of the heating device in theapparatus for forming an interconnection;

FIG. 14 is a schematic view of a polishing device in the apparatus forforming an interconnection;

FIG. 15A is a perspective view of a cleaning/drying device in theapparatus for forming an interconnection, and FIG. 15B is an enlargedview of a portion thereof;

FIG. 16 is a plan arrangement view showing another example of anapparatus for forming an interconnection;

FIG. 17 is a plan arrangement view showing another example of anapparatus for forming an interconnection; and

FIG. 18 is a cross-sectional view of a bevel/backside cleaning device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a first embodiment of the present invention, composite metallicultrafine particles are produced as follows: The surface of a core metalproduced from a metallic salt, a metallic oxide, or a metallic hydroxideand having a particle diameter of 1 to 100 nm is covered with an organiccompound including a functional group having chemisorption capabilityonto the surface of the core metal.

This organic compound serves as a protective coating for the core metal.Covering the periphery of the core metal with the organic compound(protective coating) can provide composite metallic ultrafine particleswhich are less likely to agglomerate in a solvent and have excellentdispersion stability. Moreover, the organic compound is stronglychemisorbed onto the surface of the core metal with a chemical bondingstrength. Unlike the conventional metallic ultrafine particles, whichhave been stabilized by coating with use of a surface-active agent, thecomposite metallic ultrafine particles according to the presentinvention have enhanced stability in the properties of particles and arestable even with a higher metal content.

The amount of organic compound for covering the core metal in thecomposite metallic ultrafine particles is preferably 0.01 to 1 molecule,and more preferably 0.05 to 0.5 molecule, per metal atom on the surfaceof the core metal. For example, in the case where a long-chain alkylcompound is used as the organic compound, when the core metal is silver,the amount of organic compound adsorbed is preferably about 0.25molecule per metal atom. When the core metal is copper, the amount oforganic compound adsorbed is preferably about 0.13 molecule per metalatom. The content of the metal in the ultrafine particles significantlyvaries depending upon the particle diameter and the molecular weight ofthe organic compound to be chemisorbed. However, when the practicalapplication of the composite metallic ultrafine particles, for example,to metallic coatings and catalysts is taken into consideration, it isdesirable that the composite metallic ultrafine particles have a highermetal content. Thus, the metal content is preferably 70 to 99.5% byweight, and more preferably 90 to 99.5% by weight.

The average particle diameter of the core metal is preferably 1 to 100nm, and more preferably 1 to 50 nm, and more preferably 1 to 18 nm, and,particularly, more preferably 1 to 10 nm. It has been known that themelting point of metallic particles lowers as the particle diameterdecreases. This effect appears when the particle diameter is not morethan 100 nm, and becomes significant when the particle diameter is 1 to18 nm. When the particle diameter is 1 to 10 nm, some metals begin tomelt even at around ordinary temperature. However, if the particlediameter of the core metal is excessively small, then the content of themetal in the composite metallic ultrafine particles is lowered, andhence the composite metallic ultrafine particles lose the value as ametal source. The aforementioned particle diameter permits the coremetal to melt at not more than a temperature of the melting pointinherent in the metal. For example, in the case of silver, its meltingpoint is 1233 K (960° C.), and silver ultrafine particles having adiameter of 30 nm melt at about 150° C.

Therefore, in the case where silver ultrafine particles having adiameter of 30 nm, for example, are used as the core metal, and theperiphery of the silver ultrafine particles is covered with the organiccompound, a coating consisting solely of melted silver can be formed byheating the particles to not less than the decomposition temperature ofthe organic compound.

The composite metallic ultrafine particles are produced by mixing ametallic salt, a metallic oxide, or a metallic hydroxide with an organiccompound including a functional group having chemisorption capabilityonto a core metal produced from the metallic salt, the metallic oxide,or the metallic hydroxide, and then heating the mixture under the refluxcondition of the organic compound for reaction.

For example, the composite metallic ultrafine particles may be producedas follows: A mixture 10 including a metallic salt, a metallic oxide, ora metallic hydroxide, and an organic compound (reactant) is suppliedinto a two-neck flask 12, as shown in FIG. 1. An agitation rod 14 isintroduced into the flask 12 through one of the necks of the two-neckflask 12. The mixture 10 is agitated through agitation impellers 16attached to the lower end of the agitation rod 14. A reflux condensertube 18 is mounted on the other neck, and the flask 12 is heated by amantle heater 20 to produce composite metallic ultrafine particles. Withrespect to the heating temperature and the heating time, properconditions may be selected according to the combination of the metallicsalt, the metallic oxide, or the metallic hydroxide with the organiccompound. Further, instead of heating with a heater, other means ofapplying energy, e.g., microwave heating, may be employed.

In this case, the organic compound is evaporated, and the evaporatedorganic compound is cooled and liquefied through the reflux condensertube 18 and returned to the inside of the flask 12. In this state, themetallic salt, the metallic oxide, or the metallic hydroxide isdecomposed and reduced, so that the color tone of the liquid phasewithin the flask 12 is changed to indicate the formation of ultrafineparticles. In order to promote the decomposition reduction reaction, achemical reducing agent such as hydrogen or hydrazine may simultaneouslybe used. If the heating process is performed without the reflux of theorganic compound, the metallic salt or the metallic oxide is locally orentirely evaporated to dryness and pyrolyzed to produce bulky metallicparticles. Therefore, in order to maintain and promote the reflux state,a solvent having a high boiling point, such as a naphthenic solvent, maysimultaneously be used.

Any metallic salt, metallic oxide, or metallic hydroxide may be used aslong as it can be pyrolyzed at a relatively low temperature and does notinclude such a substance that prevents the ultrafine particles frombeing produced in the reaction by heating. Preferred examples thereofinclude carbonates, nitrates, chlorides, acetates, formates, citrates,oxalates, urates, phthalates, fatty acid salts having four or lesscarbon atoms, metallic oxides, and metallic hydroxides, of Ag, Au, Bi,Co, Cu, Cr, Fe, Ge, In, Ir, Ni, Os, Pd, Pt, Rh, Ru, Si, Sn, Ti, and V.More preferred are the carbonate, the formate, the oxalate, the citrate,the oxide, and the hydroxide which produce only metals within thereaction system during pyrolysis.

On the other hand, any organic compound may be used as long as itincludes a functional group having chemisorption capability onto thesurface of the core metal produced from the metallic salt, the metallicoxide, or the metallic hydroxide, and has a boiling point that is notless than the decomposition temperature of the metallic salt, themetallic oxide, or the metallic hydroxide, and can be maintained in thereflux state under reaction temperature conditions. Preferred examplesthereof are organic compounds having a hydroxyl, carboxyl, thiol, amino,or amide group, and having four or more carbon atoms, and morepreferably eight or more carbon atoms. However, as the number ofincluded carbon atoms increases, the solubility of the organic compoundin a solvent at the time of purification is lowered, so thatsatisfactory separation and purification cannot be achieved. Therefore,the number of carbon atoms in the organic compound is preferably notmore than 22 and, more preferably, not more than 18. It is noted thatthe decomposition temperature herein does not refer to the decompositiontemperature of the metallic salt, the metallic oxide, or the metallichydroxide per se, but the decomposition temperature thereof in thepresence of the organic compound. For example, in the case of silvercarbonate, silver carbonate per se is decomposed at around 180° C., and,in the presence of stearyl alcohol, silver carbonate performs reactioneven at around 140° C. A mixture of the metallic salt, the metallicoxide, or the metallic hydroxide with the organic compound is heatedunder the reflux condition of the organic compound for reaction. At thistime, the metallic salt, the metallic oxide, or the metallic hydroxideis not necessarily required to be dissolved in the organic compound.

The amount ratio of the metallic salt, the metallic oxide, or themetallic hydroxide to the organic compound is not particularly limited.However, if the amount of organic compound is excessively small, thenthe reflux state cannot be maintained at the time of heating. In thiscase, the metallic salt, the metallic oxide, or the metallic hydroxide,or the ultrafine particles are pyrolyzed in a dry state to produce bulkymetallic particles. Therefore, the amount ratio should be such that themetallic salt, the metallic oxide, or the metallic hydroxide can bemaintained in such a state that it is sufficiently dissolved ordispersed in the organic compound. On the other hand, a portion of theorganic compound that is not used for formation of the protective layeris an impurity to be separated and removed in the purification processafter the production of the ultrafine particles. Therefore, it isunfavorable that the amount of organic compound used is excessivelylarge. From this point of view, according to the present invention,since the amount ratio of the metallic salt, the metallic oxide, or themetallic hydroxide as the metal source to the organic compound for theprotective coating can be manipulated as desired, optimal conditions canbe selected throughout the whole processes from synthesis topurification.

After completion of the reaction by heating, the composite metallicultrafine particles produced are separated from the reaction system tobe purified by a conventional method. Commonly used methods, e.g.,solvent extraction, agglomeration separation, or centrifugal separation,can be applied to the purification method. Particularly, in the casewhere the organic coating is nonpolar, when the ultrafine particles aretreated with a polar solvent such as ethanol or acetone, the ultrafineparticles are agglomerated to grow larger. On the other hand, sinceimpurities such as reaction materials or reaction products are polar,they are dissolved in the solvent. Therefore, only the ultrafineparticles covered with the organic compound can be separated byfiltration. Further, when the decomposition reaction has excessivelyproceeded, bulky metallic particles may be formed. Such bulky particlesare not dissolved in a polar solvent of the treatment. Therefore, in thecase of the separation with a filter paper, for example, the bulkyparticles are collected on the filter paper, as with the ultrafineparticles. However, since the ultrafine particles are easily dispersedin a nonpolar solvent such as toluene or hexane, the particles whichhave been collected on the filter paper are disperse in such a solventto separate the bulky metallic particles through precipitation orfiltration.

The composite metallic ultrafine particles thus produced are dispersedin a suitable organic solvent, e.g., cyclohexane, to produce adispersion liquid of composite metallic ultrafine particles. Since thedispersed particles are remarkably fine in such a dispersion liquid ofcomposite metallic ultrafine particles, the dispersion liquid issubstantially transparent in such a state that the composite metallicultrafine particles are mixed with the organic solvent and the organicsolvent is agitated. The physical properties of the dispersion liquid,such as surface tension and viscosity, can be adjusted by properlyselecting the type of the solvent, the ultrafine particle concentration,the temperature, and the like. The composite metallic ultrafineparticles thus produced are so stable as to be stored in air for a longperiod of time, after drying.

FIGS. 2A and 2B are diagrams schematically showing a structure of acomposite metallic ultrafine particle according to a second embodimentof the present invention. This composite metallic ultrafine particle 30comprises a core metal 31 essentially consisting of a metalliccomponent, and an organic compound 32 including an alcoholic hydroxylgroup which surrounds the periphery of the core metal 31. Specifically,the composite metallic ultrafine particle 30 comprises the organiccompound 32 including an alcoholic hydroxyl group, and a metalliccomponent derived from the metallic compound as the starting compound.The central portion of the composite metallic ultrafine particle isconstituted by the core metal 31, and the periphery thereof issurrounded by the organic compound 32 including an alcoholic hydroxylgroup with an alkoxide bond.

The organic compound 32 including an alcoholic hydroxyl group serves asa protective coating for the core metal 31. Thus, covering the peripheryof the core metal with the organic compound 32 including an alcoholichydroxyl group can provide composite metallic ultrafine particles whichare less likely to agglomerate in a solvent and have excellentdispersion stability.

Usually, the ratio of the core metal 31 in the composite metallicultrafine particle 30 may be about 50 to about 95% by weight. Forexample, when the composite metallic ultrafine particles are used asmetallic materials for filling interconnection grooves, the ratio of thecore metal 31 is preferably about 60 to about 95% by weight, and,particularly, more preferably 70 to 95% by weight.

The average particle diameter of the core metal 31 in the compositemetallic ultrafine particle 30 is usually about 1 to about 100 nm, andpreferably about 1 to about 20 nm, and more preferably about 5 to about15 nm. It has been known that the melting point of metallic particleslowers as the particle diameter decreases. This effect appears when theparticle diameter is not more than 100 nm, and becomes significant whenthe particle diameter is not more than 20 nm. The effect becomes moresignificant when the particle diameter is not more than 10 nm.Therefore, it is possible to melt the core metal 31 in the compositemetallic ultrafine particle 30 at not more than a temperature of themelting point inherent in the metal. For example, in the case of silver,its melting point is 1233 K (960° C.), and silver ultrafine particleshaving a diameter of 5 nm melt at about 150° C.

Therefore, in the case of a composite metallic ultrafine particle 30produced by using silver ultrafine particles having a diameter of 5 nm,for example, as the core metal 31, and covering the periphery of thesilver ultrafine particle with the organic compound 32 including analcoholic hydroxyl group, a coating consisting solely of melted silvercan be formed by heating the composite metallic ultrafine particle to150° C. when the decomposition temperature of the organic compound 32including an alcoholic hydroxyl group is not more than 150° C., or tothe decomposition temperature of the organic compound 32 including analcoholic hydroxyl group when the decomposition temperature of theorganic compound is more than 150° C.

However, as the particle diameter decreases, the amount of organiccompound covering the core metal relatively increases to lower the coremetal ratio. Thus, it is desirable that the particle diameter is largeto some extent, and hence the particle diameter is preferably 5 to 15nm.

The composite metallic ultrafine particles 30 can be produced by heatinga metallic compound, e.g., silver carbonate, in the presence of anorganic compound including an alcoholic hydroxyl group, e.g., stearylalcohol, to a temperature which is equal to or higher than thedecomposition reduction temperature of the metallic compound and isapproximately equal to the decomposition initiation temperature of theorganic compound including an alcoholic hydroxyl group. In this case, areducing agent such as acetaldehyde may be added, and then a heatingprocess may be performed to produce the composite metallic ultrafineparticles.

The metal constituting the core metal 31 may be at least one memberselected from the group consisting of Cu, Ag, Au, In, Si, Ti, Ge, Sn,Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, V, Cr, and Bi. The organic compound32 including an alcoholic hydroxyl group may be a straight-chain orbranched-chain alcohol having four or more carbon atoms, or an aromaticcompound including a hydroxyl group.

The heating temperature is approximately the decomposition initiationtemperature of the organic compound including an alcoholic hydroxylgroup and is not less than the decomposition temperature of the metalliccompound (metallic salt). For example, in the case of stearyl alcohol,its decomposition initiation temperature is 150° C. Therefore, in thiscase, the system may be held at a temperature of about 150° C. at whichthe metallic compound is decomposed. At that time, the heatingatmosphere is preferably an inert gas atmosphere. However, a heatingprocess may be performed in air for some selected combination of theorganic compound 32 including an alcoholic hydroxyl group and themetallic compound.

After completion of the heating, a purification process is performed bya conventional method. For example, the purification process may beperformed by centrifugal separation, membrane purification, or solventextraction.

The composite metallic ultrafine particles 30 thus produced aredispersed in a suitable organic solvent, e.g., cyclohexane, to prepare adispersion liquid of composite metallic ultrafine particles. Since thedispersed particles are remarkably fine in such a dispersion liquid ofcomposite metallic ultrafine particles, the dispersion liquid issubstantially transparent in such a state that the composite metallicultrafine particles are mixed with the organic solvent and the organicsolvent is agitated. The physical properties of the dispersion liquid,such as surface tension and viscosity, can be adjusted by properlyselecting the type of the solvent, the ultrafine particle concentration,the temperature, and the like.

Next, another production process of composite metallic ultrafineparticles according to the present invention will be described below.

According to this production process, a metal source is firstlydissolved or dispersed in a hydrophilic nonaqueous solvent to form asolution for composite metallic ultrafine particles. Subsequently, anorganic compound including a functional group having chemisorptioncapability onto the surface of the core metal produced from the metalsource, and the solution for composite metallic ultrafine particles isadded to a hydrophobic nonaqueous solvent to form an inverted micelle,i.e., a precursor of ultrafine particles. A reducing agent is then addedto reduce the precursor of ultrafine particles, for thereby producingcomposite metallic ultrafine particles comprising a metallic core havinga particle diameter of 1 to 100 nm and the organic compound. In thiscase, preferably, the heating atmosphere is an inert gas atmosphere inorder that the organic compound is difficult to be decomposed. However,a heating process may be performed in air for some nonaqueous solventsselected.

Thus, the organic compound including a functional group havingchemisorption capability onto the surface of the core metal producedfrom the metal source, and the solution for composite metallic ultrafineparticles is added to the hydrophobic nonaqueous solvent. The peripheryof the hydrophilic nonaqueous solvent including a metallic salt issurrounded by the organic compound including a functional group havingchemisorption capability onto the surface of the metallic particles, forthereby forming a colloid (inverted micelle) having a uniform size,i.e., a precursor of ultrafine particles.

The organic compound serves to control the particle diameter ofcomposite metallic ultrafine particles. For example, a higher alcoholhaving six or more carbon atoms, a surface-active agent, or the likeserves as a wall of the hydrophilic nonaqueous solvent including themetallic salt. Therefore, both of the two solvents provide a field forthe reduction reaction of the metallic salt without being dissolved ineach other. The reducing agent is added to reduce the metallic saltwithin the inverted micelle. Thus, composite metallic ultrafineparticles having a diameter of 1 to 100 nm can be synthesized. The sizeof the final composite metallic ultrafine particles is determined by theconcentration of the metallic salt in the field for the reductionreaction. The size of the synthesized composite metallic ultrafineparticles increases as the concentration of the metallic salt in thefield for the reduction reaction increases, and decreases as theconcentration of the metallic salt in the field for the reductionreaction decreases. The amount (weight ratio) of organic compound usedis preferably about 1 to about 50% with respect to the hydrophobicnonaqueous solvent.

The higher alcohols include lauryl alcohol and stearyl alcohol, forexample. The surface-active agents include sorbitan tristearate, forexample.

Further, the periphery of the metallic ultrafine particles is surroundedby the organic compound to form composite metallic ultrafine particleswhich are less likely to agglomerate and have excellent dispersionstability.

The ratio of the metal in the composite metallic ultrafine particle isnot particularly limited. However, the ratio of the metal may usually beabout 50 to about 95% by weight. For example, when the compositemetallic ultrafine particle is used as a metallic material for fillinginterconnection grooves, the ratio of the metal is preferably about 60to about 95% by weight, and, particularly, more preferably 70 to 95% byweight.

The average particle diameter of the metal in the composite metallicultrafine particles is usually about 1 to 20 nm, and preferably about 1to 10 nm. It has been known that the melting point of metallic particleslowers as the particle diameter decreases. This effect appears when theparticle diameter is not more than 20 nm, and becomes significant whenthe particle diameter is not more than 10 nm. Therefore, it is possibleto melt the metal in the composite metallic ultrafine particle at notmore than a temperature of the melting point inherent in the metal. Forexample, in the case of silver, its melting point is 1233 K (960° C.),and silver ultrafine particles having a diameter of 5 to 10 nm melt atabout 80° C.

Therefore, in the case of composite metallic ultrafine particlesproduced by synthesizing silver ultrafine particles having a diameter of5 to 10 nm, and covering the periphery of the silver ultrafine particlewith the organic compound, a coating consisting solely of melted silvercan be formed by heating the composite metallic ultrafine particle to80° C. when the decomposition temperature of the organic compound is notmore than 80° C., or to the decomposition temperature of the organiccompound when the decomposition temperature of the organic compound ismore than 80° C.

The metallic component in the metal source may be at least one memberselected from the group consisting of Cu, Ag, Au, In, Si, Ti, Ge, Sn,Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, V, Cr, and Bi. The organic compoundmay be an alcohol having four or less carbon atoms or a surface-activeagent.

Any inorganic salt or organometallic compound as the metal source may beused as long as it can be pyrolyzed at a relatively low temperature anddoes not include such a substance that prevents the ultrafine particlesfrom being produced in the reaction by heating. Preferred examples ofthe inorganic salt include carbonates, nitrates, and chlorides.Preferred examples of the organometallic compound include metals towhich an alkyl or allyl group, preferably an ethyl or methyl group, hasbeen bonded.

Citric acid or ascorbic acid may be used as the reducing agent, and,preferably, the system is gradually heated to a temperature at which thereduction action is developed. Although a highly hazardous or toxicinorganic reducing agent (e.g., NaBH₄ or N₂H₂) may be used as thereducing agent in some cases, the reduction can also be achieved withuse of an organic compound such as formaldehyde or acetaldehyde. Whenthe metal source is an inorganic salt, the amount (mole ratio) ofreducing agent added is preferably about 1 to 5 times as much as that ofthe inorganic salt.

More preferred reducing agents are organic acid compounds such asascorbic acid and citric acid, which are harmless natural materials.

The addition of an antioxidant enables composite metallic ultrafineparticles to be synthesized from a metal liable to be oxidized. Further,the stability of the composite metallic ultrafine particles can also beincreased to store the composite metallic ultrafine particles for a longperiod of time. For example, ascorbic acid has functions of a reducingagent and an antioxidant.

The solution obtained after reduction by heating is stable. Apurification process may be performed by a conventional method. Forexample, the purification process may be performed by precipitationseparation, column separation, centrifugal separation, membranepurification, or solvent extraction.

The dispersion liquid obtained after reduction by heating or thecomposite metallic ultrafine particles purified by the above method aredispersed in a suitable organic solvent, e.g., cyclohexane, to prepare adispersion liquid of composite metallic ultrafine particles. Since thedispersed particles are remarkably fine in such a dispersion liquid ofcomposite metallic ultrafine particles, the dispersion liquid issubstantially transparent in such a state that the composite metallicultrafine particles are mixed with the organic solvent and the organicsolvent is agitated. The physical properties of the dispersion liquid,such as surface tension and viscosity, can be adjusted by properlyselecting the type of the solvent, the ultrafine particle concentration,the temperature, and the like.

The composite metallic ultrafine particles may be used, for example, inthe so-called dual damascene process in which an interconnection of acopper or silver layer is formed in fine cavities, such asinterconnection grooves formed on the surface of a semiconductorsubstrate or vertically extended holes for communicating between layers,which are called contact holes.

FIGS. 3A through 3F show this method of forming an interconnection inorder of processes. In a semiconductor substrate W, as shown in FIG. 3A,an insulating film 2 of SiO₂ is deposited on a conductive layer 1 a on asemiconductor base 1 on which a semiconductor device is formed. Acontact hole 3 and an interconnection groove 4 are formed by lithographyetching, and a barrier layer 5 of TaN or the like is formed on theinsulating layer 2.

On the other hand, an ultrafine particle dispersion liquid L (see FIG.3B) is prepared by dispersing composite metallic ultrafine particles 30as shown in FIG. 2A in a predetermined solvent. Since the compositemetallic ultrafine particles 30 are covered with the organic compound32, the composite metallic ultrafine particles 30 are stable and lesslikely to agglomerate in the solvent.

The ultrafine particle dispersion liquid L is uniformly coated so as tocover the whole area of the surface of the substrate W on which aninterconnection is formed, as shown in FIG. 3B, for example, by a spincoating process which comprises dropping the ultrafine particledispersion liquid L at the center of the substrate W or a point somewhatdeviated from the center of the substrate W while rotating the substrateW, and stopping the dropping of the ultrafine particle dispersion liquidL at the time when the dispersion liquid L has covered the surface ofthe substrate W. A liquid film having a predetermined thickness which isdetermined by the viscosity of the ultrafine particle dispersion liquidL and by the surface tension acted between the surface of the substrateand the ultrafine particle dispersion liquid L, is thus formed on thesurface of the substrate.

Then, for example, a spin-drying (air-drying) process, in which thesubstrate W is rotated in such a state that the dropping of theultrafine particle dispersion liquid L is stopped, is performed toevaporate the solvent contained in the ultrafine particle dispersionliquid L. Thus, as shown in FIG. 3C, a composite metallic ultrafineparticle layer 6 formed by agglomeration of the composite metallicultrafine particles as the solid component is formed within the finecavity (the contact hole 3 and the interconnection groove 4) and on thesurface of the substrate.

Alternatively, a liquid film having a predetermined thickness may beformed on the surface of the substrate W on which an interconnection isformed, by the so-called dipping method in which the substrate W isdipped in the ultrafine particle dispersion liquid L. In this case, aportion of the surface of the substrate which is not required to becoated may be masked, or the ultrafine particle dispersion liquid L maybe agitated in order to fill the ultrafine particle dispersion liquid Linto the fine cavity (the contact hole 3 and the interconnection groove4), or the ultrafine particle dispersion liquid L or the substrate W maybe vibrated.

The above process of depositing the ultrafine particle dispersion liquidL onto the surface of the substrate W on which an interconnection isformed and spin-drying is repeated a plurality of times as needed. Forexample, when the fine cavity (the contact hole 3 and theinterconnection groove 4) has been filled with the composite metallicultrafine particle layer 6 to a predetermined level, as shown in FIG.3D, the depositing and drying process is stopped. Thereafter, a processof pyrolyzing and decomposing the composite metallic ultrafine particlelayer 6 is performed in a heating furnace under controlled atmosphere tomelt the metal particles in the composite metallic ultrafine particlesand bond them to each other. Thus, a metallic interconnection 7 as shownin FIG. 3E is formed.

Specifically, a heating process of the composite metallic ultrafineparticles, which comprises heating the composite metallic ultrafineparticles to 300° C. in 5 minutes, holding the temperature of 300° C.for 5 minutes, and cooling the composite metallic ultrafine particles toroom temperature in 10 minutes, is performed in an atmosphere of aninert gas, such as N₂, containing a small amount of oxygen or ozone.Then, the heating process is performed in an atmosphere of a pure inertgas, e.g., an atmosphere containing only N₂. This permits oxygen orozone to serve as a catalyst for separating the organic compound fromthe metal and hence can promote decomposition of the ultrafineparticles. Further, lampblack generated upon decomposition of theultrafine particles can be removed from the surface of the substrate bynitrogen, for example, for thereby preventing contamination of thesubstrate due to fumigation of the lampblack remaining on the surface ofthe substrate.

When the interconnection is formed with use of composite metallicultrafine particles having a metallic core of silver, it is desirablethat the heating process (baking) is firstly performed in such a statethat a nitrogen gas containing a small amount of oxygen or ozone isflowed, and a nitrogen gas containing hydrogen is flowed to form aninterconnection of pure silver through a reduction reaction withpreventing the oxidation of silver, and then the gas is switched to anitrogen gas. This can achieve efficient interconnection formation.

Heating decomposition at 450° C. or lower can reduce an adverse effectof heat on the semiconductor substrate or on a circuit formed on thesurface of the substrate.

Next, as shown in FIG. 3F, a CMP (chemical mechanical polishing) processis performed for the substrate to remove the metal and the barrier layerdeposited on the surface of the substrate. This can also removeexcessive portions of the interconnections. A multilayer interconnectionstructure may be formed by forming an insulating layer on the top of themetallic interconnection 7, providing again the structure as shown inFIG. 3A, on the insulating layer, and repeating the above processes.

An apparatus for performing the aforementioned method of forming aninterconnection will be described below with reference to FIGS. 4through 15.

FIG. 4 shows a rectangular indoor facility 120 in which an apparatus forforming an interconnection is disposed. On the ceiling of therectangular indoor facility 120, there are provided an exhaust duct 122for discharging an exhaust gas from a dispersion liquid supply section144 and a supplementary drying section 148 described later, an exhaustduct 124 for discharging an exhaust gas from a heating section 152, andan air-conditioning equipment 126 for air-conditioning a polishingsection 156 and the like. Further, an inlet/outlet port 130 for takingin and out a cassette 128 accommodating substrates W, and a controlpanel 132 are provided on a side wall of the rectangular indoor facility120.

As shown in FIG. 5, the rectangular indoor facility 120 is disposed in autility zone 134 in a clean room so that an end portion thereof ispositioned at an opening provided in a partition wall 138 for separatingthe utility zone 134 from a clean zone 136, and the inlet/outlet port130 and the control panel 132 are exposed to the clean zone 136. Both ofthe exhaust ducts 122 and 124 are connected to a single common exhaustduct 125, which is extended to the exterior of the utility zone 134.

As shown in FIG. 6, the interior of the rectangular indoor facility 120is divided into a loading/unloading section 140 having the inlet/outletport 130, a dispersion liquid supply section 144 having a dispersionliquid supply device 142 therein, a supplementary drying section 148having a supplementary drying device 146 therein, a heating section 152having a heating device 150, a polishing section 156 having a polishingdevice 154 therein, and a cleaning/drying section 159 having acleaning/drying device 158 therein. Each of the devices 142, 146, 150,154, and 158 are disposed in sequence along a direction of a flow of thesubstrates, so that the processes of forming an interconnection cancontinuously be performed. The dispersion liquid supply section 144 andthe supplementary drying device 146 have an explosion-proof constructionfrom the viewpoint of an organic solvent having explosiveness.

In this embodiment, the indoor facility 120 has one inlet/outlet port,and one cassette is accommodated in the inlet/outlet port. However, theindoor facility 120 may have two inlet/outlet ports, and cassettes maybe accommodated in each of the respective inlet/outlet port.

The pressures in the indoor facility 120 are respectively controlled intwo divisions, i.e., a cleaning division C having the loading/unloadingsection 140 and the cleaning/drying section 159, and a treatmentdivision P having the dispersion liquid supply section 144, thesupplementary drying section 148, the heating section 152, and thepolishing section 156. The pressure in the cleaning division C iscontrolled so as to be higher than the pressure in the treatmentdivision P and to be lower than atmospheric pressure. This can preventthe air in the cleaning division C from flowing out to the exterior ofthe indoor facility 120 and can prevent the air in the treatmentdivision P from flowing into the cleaning division C.

Thus, the treatment division P in which chemical mist or gas due tochemicals used for each of the processes is dispersed, and the cleaningdivision C for which a clean atmosphere is required are separated fromeach other so that the pressure in the cleaning division C is controlledto be higher than that in the treatment division P for preventing theair from flowing into the cleaning division C from the treatmentdivision P. Hence, the chemical mist or the gas can be prevented frombeing attached to the substrate after formation of an interconnection.

Preferably, measures against particles should be taken independently forthe treatment division P and for the cleaning division C.

FIGS. 7 and 8 show the dispersion liquid supply device 142 for supplyingthe ultrafine particle dispersion liquid L (see FIG. 3B) onto thesurface of the substrate W. The dispersion liquid supply device 142comprises a substrate holding portion 160 for holding and rotating thesubstrate W in such a state that the surface (front surface) ofsubstrate on which an interconnection is formed faces upwardly, and abottomed cup-shaped scattering preventive plate 162 surrounding thesubstrate W held on the substrate holding portion 160. The substrateholding portion 160 has a vacuum chuck for attracting and holding thesubstrate W on its upper surface, and is connected to the upper end of arotatable shaft 166 extending from a servomotor 164. When the servomotor164 is actuated, the substrate holding portion 160 is rotated. Thecup-shaped scattering preventive plate 162 is made of a materialresistant to an organic solvent, e.g., stainless steel.

A dispersion liquid supply nozzle 168 facing downwardly for dropping theultrafine particle dispersion liquid L is disposed above the center oran eccentric position of the surface of the substrate W held on thesubstrate holding portion 160. The dispersion liquid supply nozzle 168is connected to a free end of an arm 170. The arm 170 has a pipe thereinfor supplying a certain amount of ultrafine particle dispersion liquidL, for example, a pipe extended from a constant supply device 172 suchas a syringe pump. The pipe communicates with the dispersion liquidsupply nozzle 168.

Further, a bevel cleaning nozzle 174 inclined radially inwardly anddownwardly for supplying a cleaning liquid to a bevel portion of thesubstrate W is disposed above the peripheral portion of the substrate Wheld on the substrate holding portion 160. A plurality of backsidesurface cleaning nozzles 176 inclined radially outwardly and upwardlyfor supplying a gas or a cleaning liquid to the backside surface of thesubstrate W are disposed below the substrate W held on the substrateholding portion 160. A drain hole 162 a is provided in the bottom of thescattering preventive plate 162.

The substrate W held on the substrate holding portion 160 is rotated ata rotational speed of 300 to 500 rpm, preferably 400 to 500 rpm byactuating the servomotor 164. A certain amount of ultrafine particledispersion liquid L is dropped to a central portion of the surface ofthe substrate W from the dispersion liquid supply nozzle 168. When thesurface of the substrate W is covered with the ultrafine particledispersion liquid L, the dropping of the ultrafine particle dispersionliquid L is stopped, and hence the surface of the substrate W isuniformly coated with the ultrafine particle dispersion liquid L. Inthis case, a hydrophilic organic solvent such as methanol or acetone, ora cleaning liquid such as ethanol or isopropyl alcohol is simultaneouslysupplied to the bevel portion of the substrate W from the bevel cleaningnozzle 174, for thereby preventing the ultrafine particle dispersionliquid L from coating the outer circumferential surface and the backsidesurface of the substrate W. Further, a gas such as N₂ gas or air, or acleaning liquid similar to that supplied to the bevel cleaning nozzle174 is supplied to the backside surface of the substrate W from thebackside surface cleaning nozzles 176, so that the air flow or thecleaning liquid can prevent the backside surface of the substrate W frombeing contaminated.

A spin-drying (air-drying) process in which the substrate W is rotatedvia the servomotor 64 in such a state that the dropping of the ultrafineparticle dispersion liquid L is stopped is performed to evaporate thesolvent contained in the ultrafine particle dispersion liquid L coatingthe substrate W.

The above process of depositing the ultrafine particle dispersion liquidL onto the surface of the substrate W on which an interconnection isformed and spin-drying is repeated a plurality of times as needed. Forexample, when the fine cavity (the contact hole 3 and theinterconnection groove 4) has been filled with the composite metallicultrafine particle layer 6 to a predetermined level, as shown in FIG.3D, i.e., when the thickness of the composite metallic ultrafineparticle layer 6 reaches a predetermined value, the depositing anddrying process is stopped.

At the end of this process, the substrate W may be rotated at a higherrotational speed to promote the drying process of the solvent. An extraultrafine particle dispersion liquid L and the cleaning liquid used forcleaning the bevel portion and the backside surface of the substrate aredischarged to the exterior via the drain hole 162 a.

FIG. 9 shows the supplementary drying device 146. The supplementarydrying device 146 comprises a substrate holding portion 180 for holdinga substrate W in such a state that the surface of the substrate W facesupwardly, and a heating device 184 disposed above the substrate holdingportion 180 and having lamp heaters 182, for example.

The supplementary drying device 146 serves to dry a solvent that has notbeen evaporated by the above spin-drying process in the dispersionliquid supply device 142. When a solvent has sufficiently been dried bythe spin-drying process in the dispersion liquid supply device 142,e.g., in the case of considerably thin coating, the supplementary dryingdevice 146 is not required.

If the heating process is performed in such a state that an organicsolvent remains in a composite metallic ultrafine particle layer 6 (seeFIG. 3D) deposited on the surface of the substrate W, then voids may beformed on the bottom of the groove. Therefore, the solvent is completelydried by the supplementary drying device 146 to prevent formation ofvoids. The supplementary drying device 146 preferably heats thesubstrate W at a temperature that is lower than the decompositiontemperature of the ultrafine particles, for example, about 100° C., tothus prevent the supplementary drying device 146 from being contaminatedby decomposition of the ultrafine particles.

FIGS. 10 through 13 show the heating device 150 for heating thecomposite metallic ultrafine particle layer 6 (see FIG. 3D) to melt themetal particles and bond them to each other. The heating device 150comprises a heating plate 190 for holding and heating the substrate W insuch a state that the surface of the substrate W faces upwardly, ahousing 194 for covering the substrate W held on the heating plate 190to form a gas chamber 192 between the heating plate 190 and the housing194, and a frame 196 surrounding the peripheral portion of the heatingplate 190.

The heating plate 190 is formed of a material having a high thermalconductivity, such as aluminum or copper, in order to heat the substrateW uniformly and speedily. The heating plate 190 has a disk-like shapeand comprises therein a heater 198 and a temperature sensor 200 fordetecting a temperature of the heating plate 190. A cooling medium flowpassage 204 communicating with an introduction passage 203 forintroducing a cooling medium as coolant gas or air is formed andcommunicates with a discharge passage 206 for discharging a coolingmedium.

On the other hand, the housing 194 made of, for example, ceramics isfixed to a free end of a vertically movable arm 208. The housing 194 hasa conical recess 194 a, at its lower surface, for defining a gas chamber192 between the conical recess 194 a and the substrate W placed on aheating plate 190 when the housing 194 is moved downwardly. Further, agas supply port 194 b is provided at the central portion of the housing194. The gas supply port 194 b is connected to a gas supply pipe 210.Slit portions 194 c and pressing portions 194 d are alternately formedat a lower peripheral portion of the housing 194. Thus, when the housing194 is moved downwardly, the pressing portions 194 d are brought intocontact with the outer peripheral portion of the substrate W placed onthe heating plate 190 for thereby holding the substrate W between theheating plate 190 and the pressing portions 194 d, and defining gasdischarge ports 212 by the slit portions 194 c.

Further, a through-hole 196 a is formed in a frame 196, and a gas intakeport 214 is defined on the inner side of the through-hole 196 a. Anexhaust duct 216 which communicates with the gas intake port 214 isfixed to the lower surface of the frame 196, and an exhaust blower 218is connected to the exhaust duct 216.

Thus, a heating process which comprises placing and holding thesubstrate W on the upper surface of the heating plate 190, heating thesubstrate W to 300° C. in 5 minutes, holding the temperature of 300° C.for 5 minutes, and cooling the substrate W to room temperature in 10minutes, is performed to melt the metal particles of the compositemetallic ultrafine particles and bond them to each other. At this time,an inert gas, such as N₂ containing a small amount of oxygen or ozone isintroduced into the gas chamber 192 from the gas supply pipe 210, andthen a pure inert gas, such as N₂, is introduced into the gas chamber192 from the gas supply pipe 210. This permits oxygen or ozone to serveas a catalyst for separating the organic compound from the metal andhence can promote decomposition of the ultrafine particles. Further,lampblack generated when ultrafine particles are decomposed can beremoved from the surface of the substrate by N₂ gas, for example, forthereby preventing contamination of the substrate due to fumigation ofthe lampblack remaining on the surface of the substrate.

If the amount of oxygen or ozone to be introduced at this time is large,the composite metallic ultrafine particles tend to be adverselyoxidized. Therefore, a small amount of oxygen or ozone is sufficient tobe introduced.

When the interconnection is formed with use of composite metallicultrafine particles having a metallic core of silver, it is desirablethat the heating process (baking) is firstly performed in such a statethat a nitrogen gas containing a small amount of oxygen or ozone isflowed, and a nitrogen gas containing hydrogen is flowed to form aninterconnection of pure silver through a reduction reaction withpreventing the oxidation of silver, and then the gas is switched to anitrogen gas. This can achieve efficient interconnection formation.

FIG. 14 shows the polishing device 154 for removing an excessive metalfrom the surface of the substrate W with a chemical mechanical polishingprocess. The polishing device comprises a polishing table 222 having apolishing cloth (polishing pad) 220 constituting a polishing surface,which is attached on the upper surface of the polishing table 222, and atop ring 224 for holding the substrate W in such a manner that thesurface, to be polished, of the substrate W faces the polishing table222. The polishing table 222 and the top ring 224 are rotatedindependently of each other. While an abrasive liquid is being suppliedto the polishing cloth 220 from an abrasive liquid nozzle 226 disposedabove the polishing table 222, the substrate W is pressed against thepolishing cloth 220 under a constant pressure by the top ring 224 topolish the surface of the substrate W. The abrasive liquid supplied fromthe abrasive liquid nozzle 226 comprises an alkaline solution containingabrasive particles of fine particles, e.g., silica, suspended therein.The semiconductor wafer W is polished to a flat mirror finish by achemical mechanical polishing process of a composite effect comprising achemical polishing effect of the alkaline solution and a mechanicalpolishing effect of the abrasive particles.

When the polishing process is continuously performed with such apolishing device, the polishing capability of the polishing surface ofthe polishing cloth 220 is lowered. In order to recover the polishingcapability, a dresser 228 is provided, and a dressing process of thepolishing cloth 220 is performed at the time of the replacement of thesubstrate W to be polished, for example. In this dressing process, adressing surface (dressing member) of the dresser 228 is pressed againstthe polishing cloth 220 on the polishing table 222, and the dresser 228and the polishing table 222 are independently rotated to remove theabrasive liquid and polishing wastes attached to the polishing surfaceand to flatten and dress the polishing surface, whereby the polishingsurface is regenerated.

FIGS. 15A and 15B show a cleaning/drying device 158 for cleaning andspin-drying the polished substrate W. The cleaning/drying device 158comprises a rotatable table 341 in which arms 340 for holding thesubstrate W are radially fixed to upper ends of a rotatable shaft of therotatable table 341, a swing arm 343 having a cleaning nozzle 342 forsupplying a cleaning liquid vibrated by ultrasonic waves, for example,to the surface of the substrate W, and a gas nozzle 344 for supplying aninvert gas. A cleaning liquid is ejected from the cleaning nozzle 342onto the surface of the substrate W in such a state that the substrate Wis held horizontally on the rotatable table 341 and rotated, for therebycleaning the surface of the substrate W. Then, the substrate W isrotated at a high rotational speed of 1500 to 15000 rpm, for example, tobe spin-dried. Simultaneously, an inert gas is supplied from the gasnozzle 344, and the substrate W is heated to promote the drying process.

Here, as shown in FIG. 6, a film thickness sensor S₁ for measuring thefilm thickness of the composite metallic ultrafine particle layer 6 (seeFIG. 3D) from which the solvent has been evaporated in the supplementarydrying device 146 is disposed in the supplementary drying section 148. Afilm thickness sensor S₂ for measuring the film thickness of theheat-treated metallic interconnection 7 (see FIG. 3E) is disposed in theheating section 152. Further, as shown in FIG. 14, a film thicknesssensor S₃ for measuring the film thickness of the metallicinterconnection 7 (see FIG. 3E) during the polishing process is embeddedin the polishing table 222 of the polishing device 154.

The film thicknesses in respective processing stages are measured withthese film thickness sensors S₁ through S₃. When the measurement resultsindicate a shortage of the film thickness, the substrate is returned tothe dispersion liquid supply device 142, where the ultrafine particledispersion liquid L is uniformly coated on the substrate W so as tocover the whole surface of the substrate W on which an interconnectionis formed, and the substrate is rotated to be spin-dried (air-dried) foradjusting the film thickness to be a constant value or a desirablevalue. Simultaneously, on the basis of the measurement results of thefilm thickness, the total amount of ultrafine particle dispersion liquidL to be supplied onto the substrate, to be processed, by the dispersionliquid supply device 142 is controlled. Specifically, the number ofrepetition of the process comprising uniformly coating the ultrafineparticle dispersion liquid L on the substrate W so as to cover the wholesurface of the substrate W on which an interconnection is formed, andspin-drying (air-drying), or the amount of ultrafine particle dispersionliquid L to be coated on the substrate W is increased or decrease foradjustment. Thus, the measurement results of the film thickness are madeto reflect later processes.

In the present embodiment, an example in which the three sensors intotal are provided in each of the steps is shown. However, only onesensor may be provided. If the supplementary drying device 146 is notprovided, then a film thickness sensor S₄ for measuring the filmthickness of the composite metallic ultrafine particle layer 6 (see FIG.3D) from which the solvent has been evaporated may be provided in thedispersion liquid supply section 144, as shown by an imaginary line inFIG. 6. Further, a film thickness sensor S₅ for measuring the filmthickness of the metallic interconnection 7 (see FIG. 3E) after thepolishing process may be provided in the polishing section 156, insteadof or in addition to the film thickness sensor S₃ embedded in thepolishing table 222 of the polishing device 154, as shown by a imaginaryline in FIG. 6.

According to the apparatus for forming an interconnection thusconstructed, the cassette 128 accommodating the substrates W is firstlyintroduced into the inlet/outlet port 130. One of the substrates W istaken out of the cassette 128 and is transferred to the dispersionliquid supply device 142 in the dispersion liquid supply section 144. Indispersion liquid supply device 142, the ultrafine particle dispersionliquid L is supplied onto the surface of the substrate W and thesubstrate W is spin-dried. This operation is repeated a plurality oftimes as needed. When the thickness of the composite metallic ultrafineparticle layer 6 (see FIG. 3D) reaches a predetermined level, thesubstrate W is transferred to the supplementary drying device 146 in thesupplementary drying section 148. Next, the substrate from which thesolvent in the composite metallic ultrafine particle layer 6 has beenevaporated in the supplementary drying device 146 is transferred to theheating device 150 in the heating section 152. In the heating device150, the composite metallic ultrafine particle layer 6 (see FIG. 3D) isheated to melt the metal particles and bond them to each other, forthereby forming a metallic interconnection 7 (see FIG. 3E).

Next, the substrate on which the metallic interconnection 7 is formed istransferred to the polishing device 154 in the polishing section 156. Inthe polishing device 154, a chemical mechanical polishing process isperformed to remove an excessive metal on the surface of the substrateW. The substrate W is then transferred to the cleaning/drying device 158to be cleaned and dried. Thereafter, the substrate W is returned to thecassette 128. According to this apparatus for forming aninterconnection, a series of processes can continuously be performed.

Further, the film thicknesses are measured with the film thicknesssensors. When the sensors indicate a shortage of the film thickness, thesubstrate is returned to the dispersion liquid supply device 142 toadjust the film thickness to be a constant value or a desired value.Thus, the measurement results of the film thickness are made to reflectlater processes.

FIG. 16 shows another example of an apparatus for forming aninterconnection. In the apparatus, a dispersion liquid supply chamber234 housing a dispersion liquid supply device 142, a supplementarydrying chamber 236 housing a supplementary drying device 146, a heatingchamber 238 housing a heating device 150, a polishing chamber 240housing a polishing device 154, a cleaning/drying chamber 24 housing acleaning/drying device 158, and a plurality of stock yards (temporarystock chambers) 242 disposed at predetermined positions between thesechambers are radially provided with being centered at a central transferchamber 232 having a transfer robot 230 therein. A second transferchamber 248 having a movable robot 246 is disposed between aloading/unloading chamber 244 and the transfer chamber 232.

According to this apparatus for forming an interconnection, thedispersion liquid supply chamber 234 housing the dispersion liquidsupply device 142, the supplementary drying chamber 236 housing thesupplementary drying device 146, and the like can be unitized. Further,operations, such as supply of the dispersion liquid and supplementarydrying, can respectively be performed and combined for forming aninterconnection.

In this example, the transfer robot 230 is equipped with a plurality offilm thickness sensors S₆ in the hand portion (substrate holdingportion) 230 a. Thus, the transfer robot 230 comprises the filmthickness sensors S₆ for measuring the film thickness of the substrateduring the transfer course. This can eliminate the needs for stop orinterruption of processing the substrates and can increase throughput.

As the film thickness sensors S₁ through S₆, any sensor may be used aslong as it can measure the film thickness. However, it is desirable touse an eddy current sensor. The eddy current sensor generates an eddycurrent and detects the frequency and loss of the current that returnedafter passing through the substrate W for measuring the film thickness.The eddy current sensor is used in a non-contact manner. Further, anoptical sensor is also suitable. The optical sensor irradiates a lightto a sample and directly measures the film thickness from opticalinformation on the reflected light. The optical sensor can measure notonly the film thickness of a metallic film but also the thickness of aninsulating film such as an oxide film. The positions at which the filmthickness sensors are to be disposed are not limited to those as shownis the drawings. Of course, a desired number of sensors may be disposedat desired positions.

When the transfer robot used has a dry hand for handling a dry substrateand a wet hand for handling a wet substrate, a film thickness sensor maybe provided in either one or both of the hands.

FIG. 17 shows still another example of an apparatus for forming aninterconnection. A movable transfer robot 402 optionally equipped with afilm thickness sensor is disposed in an indoor facility 400. Theinterior of the indoor facility 400 is divided into a loading/unloadingsection 140 having an inlet/outlet port 130, a dispersion liquid supplysection 144 housing a dispersion liquid supply device 142, asupplementary drying section 148 housing a supplementary drying device146, a heating section 152 housing a heating device 150, a polishingsection 156 housing a polishing device 154, a bevel/backside cleaningsection 406 housing a bevel/backside cleaning device 404, and acleaning/drying section 159 housing a cleaning/drying device 158. Thedevices 142, 146, 150, 154, 404 and 158 are disposed in sequence along adirection of a flow of the substrates, so that the processes of formingan interconnection can continuously be performed. These devices, exceptthe bevel/backside cleaning device 404, have the same structures as thecorresponding devices described above, and hence a detailed explanationthereof is omitted here.

The bevel/backside cleaning device 404 can simultaneously perform metaletching in the edge (bevel) portion and cleaning on the backside surface(lower surface) of the substrate, and can prevent a native oxide ofcopper from growing on the surface of the substrate on which a circuitis formed. FIG. 18 shows this bevel/backside cleaning device 404. Thebevel/cleaning device 404 comprises a substrate holding portion 422disposed in a bottomed cylindrical waterproof cover 420 for horizontallyholding the substrate W with spin chucks 421 at a plurality of positionsalong the peripheral edge of the substrate in such a state that thesurface of the substrate faces upwardly and rotating the substrate at ahigh rotational speed, a central nozzle 424 disposed above the centralportion of the substrate W held by the substrate holding portion 422,and an edge nozzle 426 disposed above the peripheral portion of thesubstrate W. The central nozzle 424 and the edge nozzle 426 facedownwardly, respectively. Further, a back nozzle 428 is disposed belowan approximately central portion of the backside surface of thesubstrate W with facing upwardly. The back nozzle 428 is constituted soas to be movable in a radial direction and a vertical direction of thesubstrate W.

The movable width L of the edge nozzle 426 is such that the edge nozzle426 can be positioned at desired positions from the peripheral endportion of the substrate to the central portion of the substrate, and aset point of the movable width L can be inputted according to the sizeof the substrate W, the intended purpose, and the like. Usually, anedge-cutting width C is predetermined within the range of 2 mm to 5 mm.A cooper film or the like can be removed within the range of thepredetermined edge-cutting width C at rotational speeds that are as highas the amount of liquid flowing from the backside surface to the frontsurface can be negligible.

Next, a cleaning method with use of this cleaning device in an exampleof copper interconnections will be described below. The semiconductorsubstrate W is horizontally rotated together with the substrate holdingportion 422 in such a state that the substrate is horizontally held viathe spin chucks 421 by the substrate holding portion 422. In this state,an acid solution is supplied from the central nozzle 424 onto thecentral portion of the front surface of the substrate W. The acidsolution may contain any non-oxidative acid, such as hydrofluoric acid,hydrochloric acid, sulfuric acid, citric acid, and oxalic acid. On theother hand, an oxidant solution is supplied onto the peripheral portionof the substrate W from the edge nozzle 426 continuously orintermittently. The oxidant solution may be an aqueous solution ofozone, hydrogen peroxide, nitric acid, or sodium hypochlorite, or amixture thereof.

As a result, in the peripheral portion C of the semiconductor substrateW, the copper film or the like deposited on the front surface and theedge surface is rapidly oxidized by the oxidant solution, andsimultaneously etched by the acid solution supplied from the centralnozzle 424 and spreading over the surface of the substrate, so that thefilm is dissolved and removed. Thus, a steeper etching profile can beobtained by mixing the acid solution and the oxidant solution at theperipheral portion of the substrate, compared with cases where apremixed solution of these solutions is supplied from the nozzle. Inthis case, the etching rate of the copper is determined by theconcentrations of the respective solutions. When a native oxide ofcopper has been formed on the surface of the substrate on which acircuit is formed, the native oxide is immediately removed by the acidsolution spreading over the whole surface of the substrate due to therotation of the substrate, so that the native oxide does not grow. Thesupply of the oxidant solution from the edge nozzle 426 is stopped afterthe supply of the acid solution from the central nozzle 424, and hencethe silicon exposed to the surface can be oxidized to prevent the copperfrom attaching to the surface.

On the other hand, an oxidant solution and a silicon oxide etching agentare simultaneously or alternately supplied onto the central portion ofthe backside surface of the substrate W from the back nozzle 428. As aresult, the metallic copper attached to the backside surface of thesemiconductor substrate W, together with the silicon of the substrate,can be oxidized by the oxidant solution, and etched and removed by thesilicon oxide etching agent. It is desirable that the same oxidantsolution as supplied to the surface of the substrate is used as theoxidant solution, from the viewpoint of fewer kinds of chemicals used.Hydrofluoric acid may be used as the silicon oxide etching agent. Whenhydrofluoric acid is also used as the acid solution for the frontsurface of the substrate, the kinds of chemicals used can be decreased.When the supply of the oxidant solution is stopped earlier than thestopping of the supply of the etching agent solution, a hydrophobicsurface can be obtained. When the supply of the etching agent solutionis stopped earlier than the stopping of the supply of the oxidantsolution, a hydrophilic surface can be obtained. Thus, the backsidesurface of the substrate can be adjusted to be a surface which isrequired by later processes.

As described above, the acid solutions, i.e., the etching liquids, aresupplied onto the substrate to remove the metal ions remaining on thesurface of the substrate W. Then, pure water is supplied onto thesubstrate to replace the etching liquids with the pure water and removethe etching liquids, and substrate W is spin-dried. The removal of thecopper film within the range of the edge cutting width C in a peripheralportion of the front surface of the semiconductor substrate, and theremoval of contaminant copper on the backside surface of the substratecan simultaneously be performed. This process can be completed in 80seconds, for example. The edge cutting width in the etching process canbe determined arbitrarily (2 mm-5 mm). However, the time required forthe etching process does not depend on the cutting width.

EXAMPLE 1

Copper formate was used as a metallic salt, and 1-decanol was used as anorganic compound. Copper formate of 10 g was thoroughly premixed with1-decanol of 22 g. The mixture was then introduced in a two-neck flaskof 500 ml. The contents of the flask were heated to 200° C. by a mantleheater while being agitated with the agitation rod 14. The reflux of theorganic compound appeared at a temperature around 180° C., and the colorof the solution began to change from green to brown at a temperaturearound 185° C.

The solution was maintained at 200° C. for 15 minutes, and was thencooled. Precipitation and separation were carried out by acetoneagglomeration and toluene dispersion. The ultrafine particles of coppercovered with an organic compound thus obtained were observed with ascanning electron microscope. As a result, it was observed that theultrafine particles each had a core metal portion having a size of 5 to10 nm. Further, as a result of thermal analysis of the ultrafineparticles, it was found that the metal content was 91% by weight. Theultrafine particles were dispersed in toluene to prepare a paste of 0.1g/ml, and this paste was coated onto a glass substrate. The paste wasbaked in a nitrogen atmosphere at 300° C. to form a coating film ofcopper. Likewise, the paste may be coated onto a semiconductor substrateto form a coating film. The baking is carried out in an inert gasatmosphere under a reduced pressure of, for example, 10 Torr or lower,or even 1 Torr or lower. The baking temperature is preferably 200 to350° C., and more preferably 250 to 300° C.

EXAMPLE 2

Silver carbonate was used as a metallic salt, and myristic acid was usedas an organic compound. Silver carbonate of 10 g was reacted withmyristic acid of 10 g under heating in the same manner as in Example 1,except that the temperature was raised by the mantle heater to 230° C.The reflux of the organic compound appeared at a temperature around 100°C., and the color of the solution began to change to brown at atemperature around 230° C. The solution was maintained at thattemperature for 25 minutes. Separation and purification were carried outin the same manner as in Example 1. The ultrafine particles of silvercovered with an organic compound thus obtained were observed with ascanning electron microscope. As a result, it was observed that theultrafine particles each had a core metal portion having a size of 5 to20 nm. Further, as a result of thermal analysis of the ultrafineparticles, it was found that the metal content was 94% by weight. Theultrafine particles were dispersed in xylene to prepare a paste of 0.05g/ml, and this paste was coated onto a polyimide. The paste was baked inair at 250° C. to form a coating film of silver. If possible, the pasteis preferably baked in an inert gas atmosphere.

EXAMPLE 3

Stearyl alcohol was used as an organic compound including an alcoholichydroxyl group, and silver carbonate was used as a metal source. Thesematerials ground in a mortar were introduced into an eggplant-shapedflask having a volume of 1 liter. The contents of the flask were heatedat 150° C. for one hour. In accordance with heating, the color tone waschanged from light yellow to light brown and further to purple. At thesame time that the color was changed to purple, bubbles were generatedto cause volume expansion. After the completion of the reaction, acetonewas added to carry out precipitation purification, and the precipitatethus obtained was then dispersed in toluene. The dispersion liquid wasfiltered for purification and air-dried.

This denatured powder was observed with a transmission electronmicroscope. As a result, it was observed that the powder was constitutedby ultrafine particles having a particle diameter of about 5 to about 10nm. Further, as a result of X-ray diffraction of the powder, a metalliccore of silver was observed.

An absorption spectrum inherent in silver particles was obtained with anabsorptiometer. Further, as a result of TG/DTA measurement, it was foundthat the content of silver was 80% by weight.

The powder of composite metallic ultrafine particles was dispersed intoluene and in xylene. In both of the cases, the dispersion liquids hadno precipitate and were transparent. Specifically, it was found that thedispersion liquids were in a solubilized state.

The dispersion liquid as a composite metallic ultrafine particledispersion liquid was coated onto the surface of a substrate at a rateof 0.05 g per 1 cm². The substrate was dried and heated in a nitrogenatmosphere at about 250° C. As a result, the substrate could easily bebaked, and a coating film of silver was formed.

EXAMPLE 4

Phenol was used as an organic compound including an alcoholic hydroxylgroup, and silver carbonate was used as a metal source. These materialsground in a mortar were introduced into an eggplant-shaped flask havinga volume of 1 liter. The contents of the flask were heated at 180° C.for one hour. In accordance with heating, the color tone was changedfrom light yellow to light brown and further to purple. At the same timethat the color was changed to purple, bubbles were generated to causevolume expansion. After the completion of the reaction, acetone wasadded to carry out precipitation purification, and the precipitate thusobtained was then dispersed in toluene. The dispersion liquid wasfiltered for purification and air-dried.

These metallic ultrafine particles were applied to a substrate in thesame manner as in Example 3, except that the heating temperature waschanged to 300° C. As a result, the substrate could easily be baked, anda coating film of silver was formed.

EXAMPLE 5

Lauryl alcohol was used as an organic compound including an alcoholichydroxyl group, copper acetate was used as a metal source, andacetaldehyde was used as a reducing agent. These materials ground in amortar were introduced into an eggplant-shaped flask having a volume of1 liter. The contents of the flask were heated at 100° C. for one hour.In accordance with heating, the color tone was changed from blue tolight brown and further to red. At the same time that the color waschanged to red, bubbles were generated to cause volume expansion. Afterthe completion of the reaction, acetone was added to carry outprecipitation purification, and the precipitate thus obtained was thendispersed in toluene. The dispersion liquid was filtered forpurification and air-dried.

This denatured powder was observed with a transmission electronmicroscope. As a result, it was observed that the powder was constitutedby ultrafine particles having a particle diameter of about 10 to about15 nm. Further, as a result of X-ray diffraction of the powder, ametallic core of copper was observed.

An absorption spectrum inherent in silver particles was obtained with anabsorptiometer. Further, as a result of TG/DTA measurement, it was foundthat the content of silver was 85% by weight.

The powder of composite metallic ultrafine particles was dispersed intoluene and in xylene. In both of the cases, the dispersion liquids hadno precipitate and were transparent. Specifically, it was found that thedispersion liquids were in a solubilized state.

The dispersion liquid as a composite metallic ultrafine particledispersion liquid was coated onto the surface of a substrate at a rateof 0.05 g per 1 cm². The substrate was dried and heated in a nitrogenatmosphere at about 250° C. As a result, the substrate could easily bebaked, and a coating film of silver was formed.

EXAMPLE 6

Ethylene glycol was used as an organic compound including an alcoholichydroxyl group, and platinum chloride was used as a metal source. Thesematerials ground in a mortar were introduced into an eggplant-shapedflask having a volume of 1 liter. The contents of the flask were heatedat 180° C. for one hour. In accordance with heating, the color tone waschanged from light yellow to light brown and further to gray. At thesame time that the color was changed to gray, bubbles were generated tocause volume expansion. After the completion of the reaction, acetonewas added to carry out precipitation purification, and the precipitatethus obtained was then dispersed in cyclohexane. The dispersion liquidwas filtered for purification and air-dried.

These metallic ultrafine particles were applied to a substrate in thesame manner as in Example 5, except that the heating temperature waschanged to 300° C. As a result, the coating could easily be baked, and acoating film of platinum was formed.

EXAMPLE 7

Lauryl alcohol was used as an organic compound, and copper acetate wasused as a metal source. Copper acetate was dissolved in acetone toprepare a solution having a concentration of 5 millimoles. Toluene of500 ml, lauryl alcohol of 125 ml, and the copper acetate solution (asolution for composite metallic ultrafine particles) of 50 ml wereintroduced into an eggplant-shaped flask having a volume of 1 liter toform a precursor of ultrafine particles (an inverted micelle).L-ascorbic acid having a concentration of 11 millimoles and a volume of75 ml, which was dissolved in ethanol, was added to the inverted micelleto reduce copper acetate contained in the inverted micelle. At thattime, the contents of the flask were agitated with heating at theboiling temperature of toluene, 110° C. In accordance with agitating andheating, the color tone was changed from light green to light brown andfurther to reddish purple.

As a result of observation of an ultraviolet-visible absorption spectrumof the reddish purple solution, there was an absorption peak, inherentin a copper colloid, at about 570 nm, indicating that a copper colloidwas synthesized.

This liquid was stable by virtue of the addition of the reducing agent,and was stable in a cool dark place for about one month. Filling of aninert gas can further increase the stability.

The copper colloid concentrated by centrifugal separation was dried andobserved with scanning electron microscope. As a result, it was observedthat the powder was constituted by ultrafine particles having a particlediameter of about 50 to about 60 nm. Further, as a result of X-raydiffraction of the powder, a metallic core of copper was observed.Furthermore, the content of the metal was measured with TG/DTA(thermogravimetric analysis-differential thermal analysis), and it wasfound that the content of the metal was about 80% by weight.

The powder of composite metallic ultrafine particles was dispersed intoluene and in xylene. In both of the cases, the dispersion liquids hadno precipitate and were transparent. Specifically, it was found that thedispersion liquids were in a solubilized state.

The dispersion liquid as a composite metallic ultrafine particledispersion liquid was coated onto the surface of a substrate at a rateof 0.05 g per 1 cm². The substrate was dried and heated in a nitrogenatmosphere at about 300° C. As a result, the substrate could easily bebaked, and a coating film of copper was formed.

In this example, the amount (weight ratio) of hydrophilic nonaqueoussolvent used was 5 to 40% with respect to the hydrophobic nonaqueoussolvent, the amount (molar amount) of reducing agent added was 1 to 5times that of the metallic salt, and the amount (weight ratio) oforganic compound used was about 10 to 50% with respect to thehydrophobic nonaqueous solvent.

EXAMPLE 8

A surface-active agent was used as an organic compound, and copperacetate was used as a metal source. Copper acetate was dissolved inethanol to prepare a solution having a concentration of 5 millimoles.Xylene of 500 ml, nonionic surface-active agent (sorbitan tristearate)of 50 ml, and the copper acetate solution of 50 ml were introduced intoan eggplant-shaped flask having a volume of 1 liter to form a precursorof ultrafine particles (an inverted micelle). L-ascorbic acid having aconcentration of 11 millimoles and a volume of 100 ml, which wasdissolved in ethanol, was added to the inverted micelle. The contents ofthe flask were agitated with heating at the boiling temperature ofxylene, i.e., 150° C., to reduce copper acetate contained in theinverted micelle. In accordance with agitating and heating, the colortone was changed from light green to light brown and further to reddishpurple.

As a result of observation of an ultraviolet-visible absorption spectrumof the reddish purple solution, there was an absorption peak, inherentin a copper colloid, at about 570 nm, indicating that a copper colloidwas synthesized.

This liquid was stable by virtue of the addition of the reducing agent,and was stable in a cool dark place for about one month. Filling of aninert gas can further increase the stability.

The composite metallic ultrafine particles were applied to a substratein the same manner as in Example 7. The heating temperature in anitrogen atmosphere was 300° C. As a result, the coating could easily bebaked, and a coating film of copper was formed.

In this example, the amount (weight ratio) of hydrophilic nonaqueoussolvent used was 5 to 40% with respect to the hydrophobic nonaqueoussolvent, the amount (molar amount) of reducing agent added was 1 to 5times that of the metallic salt, and the amount (weight ratio) oforganic compound used was about 1 to about 30% with respect to thehydrophobic nonaqueous solvent.

1. A process for producing composite metallic ultrafine particles with acore metal covered by a protective layer comprising: a. Providing ametal source having a metallic component selected from the groupconsisting of metallic salts, metallic oxides, and metallic hydroxides;b. Providing an organic compound comprising 4-22 carbon atoms, whereinthe organic compound has a functional group consisting of an alcoholichydroxyl group; c. Mixing the metal source and the organic compound toform a mixture of the metal source and the organic compound; and d.Heating the mixture of the metal source and the organic compound to atemperature at which the metallic components in the metal source combineto form the core metal, and the alcoholic hydroxyl group of the organiccompound becomes bonded to a surface of the core metal, thereby formingthe core metal having a protective layer of an organic compound.
 2. Theprocess according to claim 1 wherein the core metal includes at leastone member selected from the group consisting of Ag, Au, Bi, co, Cu, Cr,Fe, Ge, In, Ir, Ni, Os, Pd, Pt, Rh, Ru, Si, Sn, Ti and V.
 3. The processaccording to claim 1 wherein the metal salt is selected from the groupconsisting of carbonates, nitrate, chlorides, acetates, formats,citrates, oxalates, urates, phthalates, and fatty acid salts that haveno more than four carbon atoms.
 4. The process according to claim 1wherein the organic compound having a functional alcoholic hydroxylgroup is a straight-chain or branched-chain alcohol or an aromaticcompound having a hydroxyl group.